Despite advances in diagnostic techniques, only 39-75% of episodes of respiratory illness in well-designed studies have an etiology identified. The introduction of polymerase chain reaction (PCR) has helped identified viral pathogens that were previously unidentified due to the poor performance of rapid antigen and culture diagnostics in older adults with influenza and respiratory syncytial virus being two of the most common viral etiologies. Unfortunately, without known etiologies, treatment and prevention strategies are missing. Hence more research is needed to identify pathogens associated with influenza-like illness and pneumonia. One potential pathogen is Influenza C. Influenza C originally called, the 1233 aberrant strain was discovered in March 1947 during routine surveillance for influenza. One influenza C challenge study done in young healthy adults, described the resulting illness to be similar to the common cold, but data is unavailable who are older and sick. Due to lack of comprehensive studies in influenza C, there is not a good description of the clinical picture of influenza C in adults, nor have any population based rates been calculated. If influenza C causes a significant number of medically attended illness including hospitalizations inclusion of influenza C in the adult vaccination program may prevent morbidity and mortality. Information on adults with respiratory symptoms during the past six influenza seasons has been prospectively collected to determine rates of hospital admissions and to perform yearly case-positive, control-negative influenza vaccine effectiveness studies. The surveillance began in November of 2006 and was completed May 2012. Recruitment began yearly in November in ended in April with the exception of the pandemic where surveillance was continuous from November 2008 through May 2010. Patients were approached if they presented with a respiratory illness and for each participating subject, both a nasal and a throat swab sample were obtained for RT- PCR. Patient questionnaires and chart review captured CDC-defined high risk conditions, symptoms, and vaccinations. Using this data we will be able to accomplish the following aims. Specific Aims: 1. Test specimens for influenza C from a population-based sample of adults =50 years of age with health care visits for acute respiratory illness prospectively enrolled over 6 years. 2. Define the clinical and demographic features of illness associated with influenza C compared with influenza A and B and other respiratory viruses 3. Estimate rates of hospitalizations associated with influenza C among adults =50 years of age.